Interest in the measurement of material properties using x-ray techniques has resulted in the development of compact, low power consumption x-ray sources for portable x-ray analytical instruments. Examples of such instruments are the hand-held x-ray fluorescence analyzers currently available from companies such as ThermoFisher Scientific Inc., Niton Analyzers, of Billerica, Mass., InnovX Systems of Woburn, Mass., and Oxford Instruments Company of Oxon, United Kingdom. In such conventional systems, however, the voltages of the x-ray sources have been generally limited because of the size requirements for the x-ray tube and the high voltage power supply, as well as the associated electrical insulation and radiation shielding requirements.
For example, as shown in FIG. 1, a portion of a conventional hand-held x-ray source may include an x-ray tube 10 within a housing 12 such that x-rays may be emitted by the x-ray tube through an x-ray output region 14 of the housing 12. The x-ray tube includes an anode end 16, a cathode end 18, and intermediate section 20 between the anode end 16 and the cathode end 18. The anode end 16 of the x-ray tube 10 includes an anode hood 22, an x-ray producing target 24, and an x-ray transmissive window 26. The cathode end 18 includes a cathode shroud 28, an electron emitter 34, and electrical connections 30 and 32 by which heater power is applied to the electron emitter 34. The intermediate section 20 may be formed of an electrical insulator such as ceramic or glass. The electrical insulator is sealed to the anode and cathode ends of the x-ray tube, thereby producing a interior region of the x-ray tube in which a vacuum can be produced and maintained.
During use, heater power is supplied to the cathode electron emitter 34, and a high voltage (e.g., 30-50 kV) is applied between the cathode end 18 and the anode end 16. The electric field produced by the applied high voltage accelerates electrons from the electron emitter through the vacuum to the x-ray producing target 24. The intensity of the x-rays produced at the target increases with increasing high voltage, electron beam current, and atomic weight of the target material. A portion of the x-rays produced in the target exit the tube via the x-ray transmission window 26, and exit the housing 12 via the x-ray output region 14 of the housing 12. The high voltage at the cathode end is typically provided as a negative high voltage (e.g., −50 kV) and the voltage potential at the anode end is typically provided at a reference ground potential of the system. This permits the anode end 16 of the tube 10 to be coupled directly to the housing 12. The x-ray tube 10 may be packaged in a hand held device that includes a high voltage power supply and a power source to drive the electron emitter.
For fixed values of the high voltage and electron current, the intensity of the x-rays at a location outside the x-ray tube decreases rapidly with increasing distance to the x-ray producing target. The x-ray intensity may be further reduced by the presence of intervening materials that scatter or absorb x-rays. Therefore, in order to maximize x-ray intensity at a given location, it is advantageous to minimize the distance from a sample or detector to the x-ray producing target and to eliminate to the extent possible any materials that scatter or absorb x-rays from the x-ray path. For these reasons, the x-ray producing target is placed as close as possible to the x-ray transmission window, and the x-ray transmission window is generally provided at an exterior surface of the housing at the output region. For example, the x-ray producing target and x-ray transmission window may be provided at a protruding portion or nose of a hand-held device, a portion of an example of which is shown in at 12 FIG. 1.
The accurate identification and quantification of elements at depths within certain materials, as well as the identification of certain heavy elements (e.g., lead and cadmium), generally requires the use of higher voltage sources (e.g., 80 to 150 kV) for x-ray production. Increasing the voltage level of the high voltage, however, generally requires that the length and diameter of the x-ray tube be increased in order to provide sufficient high voltage insulation between the anode and cathode conductors inside the vacuum envelope of the x-ray tube. Increased x-ray tube size therefore, requires an increase in the size of the hand-held x-ray inspection device. Further, providing sufficient electrical insulation between the housing and electrodes at significantly higher voltages also requires larger distances and thicker insulation. The doubling of the voltage level of a 50 kV tube, therefore, requires a substantial increase in size of a hand-held device that includes the higher voltage x-ray tube.
There remains a need, therefore, for a high voltage hand-held x-ray inspection device that is small-scale (uses a miniature x-ray source), yet is capable of operating in the range of approximately up to, for example, 150 kV.